1. Field of the Invention
This invention relates to rotating electric machines, and in particular to an air gap winding rotating electric machine in which the stator coils are rigidly and reliably disposed on the stator side.
2. Description of the Prior Art
In recent years, air gap winding rotating electric machines have become increasingly common. The main feature of this type of machine is that the stator coils are disposed in the space (i.e. the air gap) between the stator core and the rotor. In addition to making efficient use of space, placing the coil windings in the air gap has the advantage that the magnetic flux density of the stator coils is greatly increased, thereby increasing the capacity of the machine. Disposition of the coil windings in the air gap is particularly advantageous in large capacity and superconducting rotating electric machines.
However, air gap winding rotating electric machines have the drawback that reliably securing the stator coils in the air gap is difficult.
One method used in the prior art of attaching the stator coils to the stator core is to directly bond the two together with a high strength adhesive, but a bond strong enough to prevent loosening and vibration of the stator coils over long periods can not be achieved.
An air gap winding rotating electric machine of a presently existing type using a different method of disposing the stator coils is shown in FIGS. 1 and 2. FIG. 1 is a longitudinal cross-sectional view, and FIG. 2 is a cross-sectional view of the device of FIG. 1, taken along the line II--II of FIG. 1. In the figure, 1 is a stator side magnetic shield comprising a large number of metal (e.g. silicon steel) plates laminated to form a hollow cylinder disposed inside a stator frame (not shown in the figures). Numerals 2 indicate end plates restraining the end surfaces of the magnetic shield 1. Numeral 3 indicates a thin-walled electrically insulating cylinder coaxial with and substantially surrounding but not contacting a rotor 10. The insulating cylinder 3 is formed from a material of high strength, high resistance to heat, and good electrical insulating properties such as epoxy resin fiber glass or epoxy resin glass cloth. Numerals 4 indicate sets of stator coils disposed about the outer circumference of the insulating cylinder 3, each set comprising an upper group 4a and a lower group 4b separated by an axially extending electrically insulating plate 6. Between adjacent groups of stator coils 4 are first electrically insulating members 7 and second electrically insulating members 8. Each of the second electrically insulating members 8 is formed with a outward radial projection 8a extending for its entire axial length, which fits into an axially extending slot 1a formed in the inner circumferential surface of the magnetic shield 1. The first and second electrically insulating members are made of a material such as molded epoxy resin glass cloth. Numerals 5 indicate outer electrically insulating plates disposed about the outer circumference of the stator coils 4 and aligned parallel to the axis of the magnetic shield 1. Between the inner cylindrical surface of the magnetic shield 1 and the outer electrically insulating plates 5 are means for pressing the stator coils 4 against the thin-walled electrically insulating cylinder 3. In the device shown in FIGS. 1 and 2, these means comprise pairs of wedges 9 manually inserted from both axial ends of the magnetic shield 1. The wedges 9 are installed by driving them into place with a mallet until the stator coils 4 are rigidly secured by the force pressing them against the insulating cylinder 3. The wedges 9 are formed of an electrically insulating material of high strength such as epoxy resin glass cloth laminate.
During operation of an air gap winding rotating electric machine such as that shown in FIGS. 1 and 2, large lateral (i.e. in the circumferential direction) forces are exerted on the stator coils 4. These forces are partially borne by the second electrically insulating members 8. As shown in FIG. 2, lateral forces W.sub.1 and W.sub.2 are transmitted from the upper group of stator coils 4a and lower group of stator coils 4b, respectively, to the second electrically insulating members 8. These forces result in a torque about the base of the outward radial projection 8a.
This torque tends to loosen the connection between the outward radial projection 8a and the slot 1a into which it fits. Any looseness in this connection and any resulting movement of the second electrically insulating members 8 is of course undesirable, since it permits movement and vibration of the stator coils 4. Looseness of this connection may also result in damage to other parts of the rotating electric machine. If one of the second electrically insulating members 8 becomes loose, it will not be able to resist the lateral forces W.sub.1 and W.sub.2 exerted on it. Instead, these forces will be transferred to an adjacent second electrically insulating member 8 which is not loose, and these added forces may cause the latter to break.
Further, if there is looseness in the connection between the slot 1a and the outward radial projection 8a, the second electrically insulating member 8 will tend to pivot about the outward radial projection 8a when lateral forces W.sub.1 and W.sub.2 are exerted on it. In this case, the inner surface (marked B in FIG. 2) of the second electrically insulating member 8 will exert considerable force on the insulating cylinder 3, causing it to bulge inwards towards the rotor 10 and perhaps causing local failure of the insulating cylinder 3.
For these reasons, it is very important that the outward radial projection 8a tightly fits into the slots 1a in the magnetic shield 1. There must be no gaps between the sides of the outward radial projection 8a and the slots 1a.
However, a perfect fit between these two members is very difficult to achieve. Both the outward radial projections 8a and the slots 1a must be carefully manufactured in order to obtain a good fit, making manufacture costly and time consuming. Further, in the device shown in FIGS. 1 and 2, there is no means for tightening the connection between the outward radial projection 8a and the slot 1a if at some time after assembly the second electrically insulating member 8 becomes loose. Clearly, an air gap winding rotating electric machine of the type shown in FIGS. 1 and 2 is not fully satisfactory in regards to the means used for securing the stator coils 4 to the stator.